In our previous post on the topic of strengthening stone arch bridges, we outlined how arch bridges fail. Understanding how a stone arch bridge becomes overloaded helps us to find ways to strengthen such a bridge. This understanding helps to make sense of some of the techniques we will outline in this post. When it comes to strengthening a stone arch bridge, there is a surprisingly wide variety of options available, ranging from simple to elaborate. Please be aware that our descriptions here are for idea purposes only; they are not intended to spell out in detail the exact process that will be required to strengthen a stone bridge using one or more of the listed methods. Also, once reinforced, a stone bridge will still need to have its weight-carrying capabilities entirely re-evaluated to determine the maximum safe operating load.
One obvious factor directly related to any bridge’s strength is its condition. Thus, a stone arch bridge’s condition is important to its ability to handle weight. This means that the masonry should be intact, missing and misplaced stones should be reset, and mortar joints should be kept in good condition. It follows, then that one method of strengthening a stone arch bridge is repairing it as needed to obtain a good condition. Sometimes, however, something a little more than can be obtained from routine maintenance is required. Fortunately, there are several ways to strengthen a stone arch bridge significantly.
Increasing the Fill Height
One very simple method of strengthening a stone arch bridge is to increase the amount of fill over the bridge. This method works primarily by distributing the load of vehicles crossing the bridge, though it also serves to increase the amount of compression in the arch and reduces the weight difference between the crown and haunches of the arch. As outlined in the previous post of this series, arches prefer well-distributed loads as opposed to concentrated loads, which is why increasing the height of the fill over the bridge works to strengthen it. In general, fill should ideally be at least one foot deep over the arch of the bridge. That said, the load distribution caused by the fill becomes particularly effective when the fill is two or more feet deep. The popular modified MEXE method of arch bridge weight limit rating takes fill depth into account in its calculations. To implement a deeper fill on a stone arch bridge, the spandrel and approach walls will likely need to be built up higher to hold the fill material. If this is done in stone, the modification will be quite elegant when completed.
This is one of the simplest methods of strengthening a stone arch bridge. Though it has its limitations, overall it is surprisingly effective.
Distributing Loads with a Reinforced Concrete Slab
While increasing the height of the fill over a stone arch bridge is effective for increasing its weight-carrying capacities, adding a reinforced concrete slab to the top of the bridge strengthens the bridge even more. The reinforced concrete slab does what the increase of fill height does, only better. As a reinforced concrete slab can handle significant tensile forces, it is better able to distribute a moving load over the entire arch. This makes the load distribution highly effective. According to information published in the excellent book Safety of Historical Stone Arch Bridges by Dick Proske and Pieter van Gelder, experiments revealed that the load capacity of a stone bridge could be increased by a factor of over three times simply by adding a reinforced concrete slab to the structure. Such an increase in weight-carrying capacity is huge, though in practice a detailed computation is necessary to confirm the final strength of the stone arch bridge once thus modified. The same book also noted that German railways allowed for an automatic increase of 1.2 times the original weight-carrying capacity of a stone arch bridge once the bridge was modified with a reinforced concrete slab without any detailed calculations of weight-bearing capacity. Not only is the addition of a reinforced concrete slab quite effective for updating existing stone arch bridges to conform to modern requirements, but it can also be done totally unobtrusively. This makes the reinforced concrete slab an excellent solution for strengthening historic stone arch bridges where alterations to the bridge’s original form are unacceptable. Where function is a little more important than form, reinforced concrete slabs can also be cantilevered over the sides of the stone arch bridge to widen it.
The final result of adding a cantilevered concrete slab to a stone bridge is a wider, stronger structure. Such a modification will likely be much less expensive than building a new bridge, and allows longer road service for old, outdated stone bridges.
Replacing the fill with concrete can also be used to strengthen a stone arch bridge. Concrete is a solid mass, and distributes loads well. Furthermore, a solid fill such as concrete is effective at preventing individual arch stones from working loose, even under overload conditions. A further benefit of concrete fill is the fact that it puts little strain on spandrel walls, making the spandrel walls far more stable. Replacing the fill with concrete prevent spandrel walls from sliding out and failing. It should be noted, however, that some stone arch bridges rely on the fill to press the arch stones together. This is especially true for bridges employing a Roman arch. The upshot, then is that falsework may need to be added under the arch of the bridge during the excavation of the fill required to replace it with concrete. It is worth pointing out that, if the concrete fill is poured deep enough to completely cover the entire existing arch with a thick layer of concrete, the concrete fill can begin to act as an arch in its own right, strengthening the bridge that much more.
Internal Concrete Arch
A next step beyond adding a concrete fill to a stone bridge is to pour a reinforced concrete arch inside the bridge directly over the arch. Commonly known as a “saddle,” such reinforcement is essentially a way of thickening the arch, and is unobtrusive and highly effective. Thickening the arch works to strengthen the bridge by increasing the area within the arch ring the thrust of a load has to work in. Furthermore, if the internal arch is made of reinforced concrete, it can handle some tensile forces as well. An internal concrete arch is often used in conjunction with the previously mentioned method of replacing the fill with concrete; after all, all the fill will need to be excavated anyway in order to pour a concrete saddle. Remember, some stone arch bridges will need to have the arch supported when the fill is removed.
External Arch Thickening
A related method of reinforcing a stone arch bridge’s arch is to pour an external concrete arch beneath the stone arch. The concrete is usually sprayed on (shotcrete), though, conceivably, it could be poured in a conventional manner with some difficulty. A related method is to use an arch liner, which works just like the external concrete arch, only is made of metal. Both metal and concrete liners can quite easily cause water damage to the arch if proper drainage is neglected.
An external arch below the stone arch results in an obvious external alteration to the bridge, and is ideally avoided if possible. That said, there have been some bridges excellently restored with the external concrete arch, including the oldest stone arch bridge in the country, which was erected in 1697 and is in Philadelphia.
There are stone arch bridge reinforcement methods put forth by various companies. They appear to be effective, and usually involve reinforcing the arch in some fashion with a material strong in tension, to allow for tensile forces in the arch. The material is usually metal, but can be other substances such as carbon fiber reinforced polymer. These methods can be quite effective, and can almost always be tucked away within the bridge out of site.
One other, extreme method of “reinforcing” a stone arch bridge actually consists of removing load from the original bridge altogether. The stone arch bridge is retained for looks while a new bridge that does all the work is added over or even in the stone bridge’s fill. This method, called overbridging, will likely require some ingenuity for some details such as pier placement, for instance. Piers can usually be placed through the fill of the stone arch bridge at either end of the arch, and, with a little planning, the final result can be quite effective, both for traffic purposes and for retaining the old bridge. There is another advantage to overbridging as well. Most bridges require extensive falsework to support unset concrete and the like. When overbridging, the existing stone bridge can substitute for much of the expensive formwork, potentially making overbridging more economical. When and how overbridging is to be carried out is a bit of a judgment call. Obviously, the original design of the stone bridge will greatly effect how the new bridge is designed. However, with a little ingenuity, overbridging can not only be quite successful, but can also be done totally unobtrusively. The new bridge can sometimes be tucked away in the fill above the stone arch bridge such that little evidence of the modification is visible.
There is a surprising variety of ways to strengthen a stone arch bridge with insufficient weight-carrying abilities. We have listed some common methods used that give an idea of all the possibilities available, and it is not uncommon to find bridges where several of these methods have been used simultaneously. When the bridge has been reinforced, some calculations will, of course, be required to determine the final safe load limit for the bridge. These methods of reinforcement allow for an insufficient stone arch bridge to be retained, while allowing it to continue to serve traffic. Sometimes, too, reinforcing the existing bridge can be quite a bit less expensive than building a new one. Stone arch bridges are generally not becoming more common, and reinforcing them is a good way to breathe new life into picturesque, historic structures.